IEEE 802.11 is a set of media access control (MAC) and physical layer (PHY) specification for implementing wireless local area network (WLAN) communication, called Wi-Fi, in the unlicensed (2.4, 3.6, 5, and 60 GHz) frequency bands. The standards and amendments provide the basis for wireless network products using the Wi-Fi frequency bands. Wi-Fi plays an important role in the growing application of Indoor Location. The key applicable Wi-Fi technology is that of ranging using time-of-flight (TOF) ranging measurements defined in IEEE 802.11v. Once the distance between devices is measured, the information can be used to determine device location.
In IEEE 802.11-REVmc, Fine Timing Measurement (FTM) protocol is proposed for Indoor Location. Based on FTM, an initiating station (STA) exchanges FTM frames with a responding station (STA) to measure the time-of-flight (TOF) or the Round Trip Delay (RTD/2). The initiating station then computes its range to the responding station after receiving timing measurements (i.e., timestamps corresponding to the departure time and arrival time of the FTM frames) from the responding station. To calculate a station position, the STA performs ranging measurements with multiple access points (APs) via FTM frame exchange and obtains AP's positions. FTM positioning requires the initiating station to exchange FTM frames with multiple responding APs for TOF measurements in order to determine its absolute location. For 3D positioning, the station needs to exchange FTM frames with four APs in order to determine its absolute location.
As well known in the teaching of navigation science, the STA positioning error, obtained from processing the timing measurements, is related to the geometry of the AP positions. This is typically describes by a parameter called PDOP (Position of Dilation). The PDOP is inversely proportional to the volume of tetrahedron formed by unit directional vectors between AP and STA positions. To achieve low positioning error, it is necessary that APs be deployed at strategically selected positions within a coverage area. Specifically, co-located basic service sets (BSSs) (e.g., BBSs on the same physical device or BSSs configured with the same location as a neighboring BSS) should not be used for FTM positioning. STA should know which BSSs are co-located to avoid initiating FTM with those APs managing the co-located BSSs. Configuring APs that share the same location with an mBSSID or LocationID element may not always be feasible. First, it requires an admin to manually configure these elements in the beacons of co-located BSSs. Second, it hardly works in a public environment with APs independently operated by different business entities. A solution for avoiding FTM with co-located BSSs is sought.